You have to go to your database and edit it manually. First, you can remove multiple entries from the history by deleting entries in bfauction_plus_bid table. Second, in the bfauction_plus table edit the currentBid and highBidder fields to its correct values.

Disregard what I've said above, do the following instead. In Edit Auction Item (just below the Description field) there is a Bid History link. It should show the bid history and Retract Bid button for each individual bids.